Modified Mercalli Intensity Scale (MMI)

The Modified Mercalli Intensity (MMI) scale depicts shaking severity. An earthquake has a single magnitude that indicates the overall size and energy released by the earthquake. However, the amount of shaking experienced at different locations varies based on not only that overall magnitude, how far you are from the fault that ruptured in the earthquake, and whether you are on rock or thick valley deposits that shake longer and harder than rock.

While newer buildings in the Bay Area are designed to withstand this shaking component without killing people, the buildings can still be heavily damaged and may not be able to be used for days, weeks, or even months after the earthquake. The descriptions of the various MMI categories that follow focus on damage to older unreinforced masonry buildings, chimneys, and wood-frame homes.

MMI Value

Summary Damage Description Used on Maps

Description of Shaking Severity

Full description shortened from Elementary Seismology

I

Not mapped

Not mapped

Not felt.

II

Not mapped

Not mapped

Felt by people sitting or on upper floors of buildings.

III

Not mapped

Not mapped

Felt by almost all indoors. Hanging objects swing. Vibration like passing of light trucks. May not be recognized as an earthquake..

Poorly built structures destroyed with their foundations. Even some well-built wooden structures and bridges heavily damaged and needing replacement. Water thrown on banks of canals, rivers, lakes, etc.

XI

Not mapped because these intensities are typically limited to areas with ground failure.

Rails bent greatly. Underground pipelines completely out of service.

XII

Not mapped because these intensities are typically limited to areas with ground failure.